Polyurethanes have been known for a long time and are distinguished by their great diversity. A review of polyurethanes, their properties and applications is given e.g. in Kunststoffhandbuch, volume 7, Polyurethane, 3rd revised edition, volume 193, edited by Prof. Dr. G. W. Becker and Prof. Dr. D. Braun (Carl-Hanser Verlag, Munich, Vienna).
As well as flexible and rigid foams, non-foamed solid polyurethanes, such as e.g. cast elastomers, are also of interest. In particular, solid polyurethanes or polyurethanes with a bulk density of >500 kg/m3 are employed in sectors where, in addition to outstanding material properties, antistatic and electrically conductive properties are also important. Sectors to be mentioned here are floor coverings, tires, paintable rolls, rollers and electrical encapsulation materials. Particularly in some highly sensitive technical equipment, it is essential that charges be avoided. Like most thermoplastic polymers, polyurethanes are not conductive per se. Common surface resistances are in the range of 1013 ohms.
Numerous additives are known to reduce this high resistance. Salts, such as e.g. ammonium salts (e.g. CATAFOR from Rhodia GmbH), were used very early on to reduce the surface resistance. Unfortunately, these additives have the disadvantage of accelerating hydrolysis in polyester polyol based polyurethanes. Moreover, migration to the surfaces and associated chalking are a great disadvantage. Added to this is the fact that the effects achieved are comparatively small and the surface resistance is reduced only by 2-3 powers of ten.
As well as the use of these salts, the use of conductive carbon black (e.g. conductive carbon black with a surface area of 600-1200 m2/g according to BET; e.g. known. The use of conductive carbon black is described e.g. in EP-A 0 129 193 and DE-A 3 528 597. Good surface resistances can be achieved with conductive carbon blacks in foamed and unfoamed polyurethanes (up to 104 ohms). However, the quantities of conductive carbon black required always lead to a very high viscosity of the reaction components, so that these systems can no longer be processed with the common polyurethane machines. These systems are therefore hardly ever used industrially. Substantially lower viscosities can be achieved by using carbon fibers—as described in DE-A 19 858 825. With relatively high concentrations of carbon fibers, surface resistances of less than 104 ohms can be achieved with processing viscosities that are just acceptable. However, in application it is shown that the fibers break in parts subject to mechanical stress and the conductivity decreases very rapidly until a non-conductive polyurethane is present again. This breaking of the fibers already occurs during processing, and so PU systems of this type are not used on an industrial scale.
Furthermore, the use of graphites (e.g. COND 8/96 from Graphit Kopfmühl AG) is conceivable for the reduction of the electrical resistance. However, to obtain a useful conductivity, concentrations would be required in the polyurethane reaction system that would mean a considerable increase in viscosity and therefore would rule out industrial processing.